Device and method for controlling distribution of drive force of four-wheel drive car

ABSTRACT

A driving force distribution control apparatus and driving force distribution control method for a four-wheel drive vehicle which can quickly increase torque distribution to sub-drive wheels after a transmission has been shifted down. According to the apparatus and method, when it is determined that the transmission has been shifted down, an ECU controls a coupling capable of changing the torque distribution to the front wheels and rear wheels such that the torque distribution to the front wheels and rear wheels approaches a uniform state.

TECHNICAL FIELD

[0001] The present invention relates to a driving force distributioncontrol apparatus and driving force distribution control method for afour-wheel drive vehicle.

BACKGROUND ART

[0002] A part-time driving system which selectively switches between afour-wheel drive (4WD) mode and a two-wheel drive (2WD) mode and afull-time driving system which always drives four wheels are known asfour-wheel driving systems for vehicles. In the part-time drivingsystem, mode switching is according to manual operation by a driver. Thefull-time driving system has a center differential between the frontwheels and rear wheels. The center differential permits the differentialof the front wheels and the rear wheels to always drive the four wheels.

[0003] A standby driving system is known too as a four-wheel drivingsystem other than the part-time driving system and full-time drivingsystem. In this standby driving system, the main drive wheels of avehicle are coupled to the engine without the intervention of thecoupling and the sub-drive wheels are coupled to the engine via thecoupling. The coupling force (engaging force) of the coupling is changedin accordance with the road conditions and driving state, therebychanging the torque distribution ratio D_(SUB/MAIN), which is the ratioof the torque to be transmitted to the sub-drive wheels from the engineto the torque to be transmitted to the main drive wheels from theengine.

[0004] In the standby driving system, a controller compares a wheelspeed difference ΔN, which is the difference between the rotationalspeed of the main drive wheels and the rotational speed of the sub-drivewheels, with a predetermined threshold value. When the wheel speeddifference ΔN exceeds the threshold value, the controller predicts thatacceleration takes place and controls the coupling in such a way thatthe torque distribution ratio D_(SUB/MAIN) increases, i.e., the torquedistribution of the sub-drive wheels increases.

[0005] Even if the transmission is shifted down to accelerate thevehicle in the standby driving system, however, the controller cannotdetect that acceleration is taking place until the wheel speeddifference ΔN exceeds the threshold value. Therefore, there is a delayfrom the point at which shift-down has been performed to the point atwhich the increase in torque distribution ratio D_(SUB/MAIN) starts.

[0006] Japanese Laid-Open Patent Publication No. 6-288415 discloses amechanical coupling as the coupling that is used in a four-wheel drivingsystem. Even if the mechanical coupling is adapted to a standby drivingsystem, however, the beginning of the increase in torque distributionratio D_(SUB/MAIN) cannot be interlocked with the shift-down.

DISCLOSURE OF THE INVENTION

[0007] It is therefore an object of the present invention to provide adriving force distribution control apparatus and a driving forcedistribution control method which can promptly increase torquedistribution to sub-drive wheels after the transmission is shifted down.

[0008] To achieve the object, the present invention provides thefollowing driving force distribution control apparatus for a four-wheeldrive vehicle. The four-wheel drive vehicle has an engine, front wheelsand rear wheels to be driven by torque generated by the engine, acoupling capable of changing torque distribution to the front wheels andrear wheels, and a transmission. The driving force distribution controlapparatus has a controller which controls the coupling in such a waythat the torque distribution to the front wheels and rear wheelsapproaches a uniform state when having determined that the transmissionis shifted down.

[0009] The present invention also provides a driving force distributioncontrol method for a four-wheel drive vehicle. The four-wheel drivevehicle has an engine, front wheels and rear wheels to be driven bytorque generated by the engine, a coupling capable of changing torquedistribution to the front wheels and rear wheels, and a transmission.The driving force distribution control method has a step of determiningwhether or not the transmission is shifted down; and a step ofcontrolling the coupling in such a way that the torque distribution tothe front wheels and rear wheels approaches a uniform state when it isdetermined that the transmission is shifted down.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a schematic structural diagram illustrating a four-wheeldrive vehicle according to one embodiment of the invention;

[0011]FIGS. 2A and 2B are graphs showing maps to be used in normal mode;

[0012]FIGS. 3A and 3B are graphs showing maps to be used in accelerationmode; and

[0013]FIG. 4 is a flowchart illustrating procedures of control, that areexecuted by an ECU equipped in the four-wheel drive vehicle in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] One embodiment of the invention will be described below referringto FIGS. 1 to 4.

[0015] A vehicle (four-wheel drive vehicle) 11 shown in FIG. 1 has anengine 12 as an internal combustion engine and a transaxle 13. Thetransaxle 13 has a manual transmission and a transfer (both not shown).The manual transmission includes gears which are switched by theoperation of an unillustrated shift lever. The transaxle 13 is coupledto a pair of front axles 14 and a propeller shaft 15. Each front axle 14is coupled to a front wheel 16.

[0016] The propeller shaft 15 is coupled in a disconnectable manner to adrive pinion shaft (not shown) via a driving force transmittingapparatus, which is a coupling 17. The drive pinion shaft is coupled toa rear differential 19. The rear differential 19 is coupled to a pair ofrear axles 20. Each rear axle 20 is coupled to a rear wheel 21.

[0017] The front wheels 16 function as main drive wheels to be coupledto the engine 12 without the intervention of the coupling 17. The rearwheels 21 function as sub-drive wheels to be coupled to the engine 12via the coupling 17.

[0018] The torque (driving force) generated by the engine 12 istransmitted to the front wheels 16 via the transaxle 13 and front axles14. When the propeller shaft 15 and the drive pinion shaft are coupledtogether via the coupling 17, the torque of the engine 12 is alsotransmitted to the rear wheels 21 via the propeller shaft 15, the drivepinion shaft, the rear differential 19 and the rear axles 20.

[0019] The coupling 17 has an electromagnetic clutch mechanism 18 of awet multiplate type. The electromagnetic clutch mechanism 18 has aplurality of clutch plates (not shown), which are frictionally engagedwith or disengaged from one another, and an electromagnetic coil (notshown). As the current is supplied to the electromagnetic coil, theclutch plates are frictionally engaged with one another with forceaccording to the amount of the supplied current, resulting in that theelectromagnetic clutch mechanism 18 can transmit the torque of theengine 12 to the rear wheels 21. When the current is not supplied to theelectromagnetic coil, the clutch plates are disengaged from one another,so that the electromagnetic clutch mechanism 18 cannot transmit thetorque of the engine 12 to the rear wheels 21.

[0020] The vehicle 11 has an ECU 42 which serves as a controller. TheECU 42 controls the amount of the current supplied to theelectromagnetic coil. The amount of the torque to be transmitted to therear wheels 21 from the engine 12 varies according to the magnitude ofthe engaging force of the clutch plates of the electromagnetic clutchmechanism 18. The magnitude of the engaging force of the clutch plateschanges according to the amount of the current that is supplied to theelectromagnetic coil of the electromagnetic clutch mechanism 18.

[0021] The ECU 42 selectively switches between a four-wheel drive modein which the torque of the engine 12 is transmitted to both the frontwheels 16 and rear wheels 21, and a two-wheel drive mode in which thetorque of the engine 12 is transmitted only to the front wheels 16 butnot to the rear wheels 21. In four-wheel drive mode, the ECU 42 controlsthe coupling 17 in such a way as to change the ratio of the torque to betransmitted to the front wheels 16 from the engine 12 and the torque tobe transmitted to the rear wheels 21 from the engine 12. In other words,the ECU 42 controls the coupling 17 in such a way as to change a torquedistribution ratio D_(SUB/MAIN), which is the ratio of the torque to betransmitted to the rear wheels 21 from the engine 12 to the torque to betransmitted to the front wheels 16 from the engine 12.

[0022] When the amount of the current to be supplied to theelectromagnetic coil is zero, the transmission of the torque of theelectromagnetic clutch mechanism 18 is inhibited, resulting in that thetorque distribution ratio D_(SUB/MAIN) becomes 0/100. When theelectromagnetic clutch mechanism 18 is coupled completely, i.e., whenthe engaging force of the clutch plates is maximum, the torquedistribution ratio D_(SUB/MAIN) becomes 50/50 unless none of the fourwheels are slipping. In accordance with a control instruction from theECU 42, the torque distribution ratio D_(SUB/MAIN) is changed to withinthe range of 0/100 to 50/50.

[0023] The ECU 42 mainly comprises a microcomputer having a CPU, RAM,ROM and I/O interface. Stored in the ROM are, for example, variouscontrol programs the ECU 42 executes, and various data and maps. Themaps have been acquired beforehand by, for example, experimental databased on vehicle models and well-known theoretical computations or thelike. The RAM is a data work area for the CPU to load a control programwritten in the ROM and execute various arithmetic operations.

[0024]FIGS. 2A to 3B show the maps stored in the ROM as graphs. The mapsshown in FIGS. 2A and 2B are used when the vehicle 11 is notaccelerated, i.e., at the time the torque distribution ratioD_(SUB/MAIN) is determined to be in normal mode. The torque distributionratio D_(SUB/MAIN) is determined to be in normal mode by adding a firstcalculation value, which is computed using the map shown in FIG. 2A, anda second calculation value, which is computed using the map shown inFIG. 2B, by a predetermined ratio. The maps shown in FIGS. 3A and 3B areused when the vehicle 11 is accelerated, i.e., at the time the torquedistribution ratio D_(SUB/MAIN) is determined to be in accelerationmode. The torque distribution ratio D_(SUB/MAIN) is determined to be inacceleration mode by adding a third calculation value, which is computedusing the map shown in FIG. 3A, and a fourth calculation value, which iscomputed using the map shown in FIG. 3B, by a predetermined ratio. Eachof the first to fourth calculation values represents the magnitude ofthe torque distribution to the rear wheels 21.

[0025] The first calculation value is computed from a vehicle speed Vand a wheel speed difference ΔN based on the map shown in FIG. 2A. Thewheel speed difference ΔN is the difference between the average of therotational speed of the left and right front wheels 16 and therotational speed of the left and right rear wheels 21. When the vehiclespeed V is constant, the first calculation value linearly increases asthe wheel speed difference ΔN becomes larger. When the wheel speeddifference ΔN is constant, the first calculation value becomes smalleras the vehicle speed V becomes greater.

[0026] The second calculation value is computed from the vehicle speed Vand throttle opening Od based on the map shown in FIG. 2B. The throttleopening Od is the opening of a throttle valve (not shown) provided inthe engine 12. When the vehicle speed V is constant, the secondcalculation value increases as the throttle opening Od becomes larger.The amount of increase in the second calculation value originated froman increase in throttle opening Od becomes smaller as the throttleopening Od becomes greater. When the throttle opening Od is constant,the second calculation value becomes larger as the vehicle speed Vbecomes lower.

[0027] The third calculation value is computed from the vehicle speed Vand the wheel speed difference ΔN based on the map shown in FIG. 3A.Under the conditions of the same vehicle speed V and wheel speeddifference ΔN, the third calculation value acquired based on the mapshown in FIG. 3A is greater than the first calculation value acquiredbased on the map shown in FIG. 2A. When the vehicle speed V is constant,the third calculation value linearly increases as the wheel speeddifference ΔN becomes larger. When the wheel speed difference ΔN isconstant, the third calculation value becomes smaller as the vehiclespeed V becomes larger.

[0028] The fourth calculation value is computed from the vehicle speed Vand the throttle opening Od based on the map shown in FIG. 3B. Under theconditions of the same vehicle speed V and throttle opening Od, thefourth calculation value acquired based on the map shown in FIG. 3B isgreater than the second calculation value acquired based on the mapshown in FIG. 2B. When the vehicle speed V is constant, the fourthcalculation value increases as the throttle opening Od becomes larger.The amount of increase in the fourth calculation value originated froman increase in throttle opening Od becomes smaller as the throttleopening Od becomes greater. When the throttle opening Od is constant,the fourth calculation value becomes larger as the vehicle speed Vbecomes lower.

[0029] As shown in FIG. 1, the ECU 42 is connected to two front-wheelspeed sensors 43 a, two rear-wheel speed sensors 43 b, a throttleopening sensor 47 as a throttle opening detection device, and a gearposition sensor 48 as a gear position detection device. The ECU 42receives inputs (detection signals) from the front-wheel speed sensors43 a, 43 b, the throttle opening sensor 47 and the gear position sensor48 via the I/O interface. The ECU 42 sends an output to the coupling 17and an engine control apparatus (not shown) via the I/O interface.

[0030] The front-wheel speed sensor 43 a is provided on each of thefront wheels 16 and detects the rotational speed of the correspondingfront wheel 16. The rear-wheel speed sensor 43 b is provided on each ofthe rear wheels 21 and detects the rotational speed of the correspondingrear wheel 21. The throttle opening sensor 47, connected to the throttlevalve, detects the throttle opening Od. The throttle opening Od reflectsthe depression amount of the accelerator pedal (not shown) of thevehicle 11. The gear position sensor 48 detects the gear position of themanual transmission.

[0031]FIG. 4 is a flowchart illustrating procedures of control, whichare executed by the ECU 42. The routine shown in the flowchart of FIG. 4is executed in a predetermined cycle.

[0032] In step S1, the ECU 42 receives detection signals from thefront-wheel speed sensors 43 a, 43 b, the throttle opening sensor 47 andthe gear position sensor 48.

[0033] In step S2, the ECU 42 computes the vehicle speed V and wheelspeed difference ΔN based on the detection signals input in step S1. Thevehicle speed V is computed from the average of the rotational speeds ofthe left and right rear wheels 21.

[0034] In step S3, the ECU 42 determines whether or not the throttleopening Od is greater than a predetermined opening threshold value Os.When it is determined that the throttle opening Od is equal to or lessthan the opening threshold value Os, the ECU 42 proceeds to step S6, andwhen it is determined that the throttle opening Od is larger than theopening threshold value Os, the ECU 42 proceeds to step S4. The openingthreshold value Os is stored in the ROM beforehand as a function of thevehicle speed V. The opening threshold value Os is a relatively smallvalue and is a value equivalent to the throttle opening at the time ofengine braking, in other words, a value equivalent to the throttleopening when the depression amount of the accelerator pedal is zero ornearly zero.

[0035] In step S4, the ECU 42 determines whether or not the amount ofchange in the rotational speed of each of the wheels 16, 21 in apredetermined time (e.g., several hundred ms) is greater than apredetermined change threshold value. When it is determined that theamount of change in the rotational speed of every wheel 16, 21 is equalto or smaller than the change threshold value, the ECU 42 goes to stepS5, and when it is determined that the amount of change in therotational speed of at least one wheel 16, 21 is greater than the changethreshold value, the ECU 42 goes to step S7. The change threshold valueis stored in the ROM beforehand.

[0036] In step S5, the ECU 42 determines whether or not the transmissionhas been shifted down. In other words, the ECU 42 determines whether ornot the gear position detected in the present routine is a lower-gearposition, as compared with the gear position detected in the previousroutine. The ECU 42 goes to step S6 when it is determined that thetransmission has not been shifted down, and goes to step S7 when it isdetermined that the transmission has been shifted down.

[0037] In step S6, the ECU 42 sets a flag showing the normal mode whenthe current mode is the normal mode and sets a flag showing theacceleration mode when the current mode is the acceleration mode. TheECU 42 selects maps to be used according to the set flag and determinesthe torque distribution ratio D_(SUB/MAIN) based on the selected maps.

[0038] In normal mode, the ECU 42 determines the torque distributionratio D_(SUB/MAIN) based on the maps shown in FIGS. 2A and 2B. Inacceleration mode, the ECU 42 determines the torque distribution ratioD_(SUB/MAIN) based on the maps shown in FIGS. 3A and 3B. In accordancewith the determined torque distribution ratio D_(SUB/MAIN), the ECU 42determines the amount of the current to be supplied to theelectromagnetic coil of the electromagnetic clutch mechanism 18.

[0039] In step S7, the ECU 42 sets the flag showing the accelerationmode. And, the ECU 42 determines the torque distribution ratioD_(SUB/MAIN) based on the maps shown in FIGS. 3A and 3B, and determinesthe amount of the current to be supplied to the electromagnetic coil inaccordance with the determined torque distribution ratio D_(SUB/MAIN).

[0040] The flags that have been set in step S6 and step S7 are clearedwhen it is determined that the throttle opening Od is equal to or lessthan the opening threshold value Os in step S3 in the next routine orany one following it.

[0041] When it is determined that the throttle opening Od is greaterthan the opening threshold value Os, even if it is determined that theamount of change in the rotational speed of every wheel 16, 21 is equalto or smaller than the change threshold value, the ECU 42 enters theacceleration mode as long as it is determined that the transmission hasbeen shifted down. In acceleration mode, the ECU 42 controls thecoupling 17 in such a way that the torque distribution to the frontwheels 16 and rear wheels 21 approaches a uniform state.

[0042] Conventionally, in the case where the ECU 42 goes to theacceleration mode, the depression of the accelerator pedal by the driverchanges the rotational speed of the wheels, after which the frictionalcoupling force of the coupling 17 is changed for the first time when thewheel speed difference ΔN exceeds the threshold value. Even if thedriver depresses the accelerator pedal, therefore, the strong torquefeeling originated from an increase in torque distribution to the rearwheels 21 is not obtained soon.

[0043] According to the embodiment, by way of contrast, when the ECU 42detects shift-down which is to be carried out to accelerate the vehicle11, the ECU 42 determines the amount of the current to be supplied tothe electromagnetic coil of the electromagnetic clutch mechanism 18 andsupplies the determined amount of current to the electromagnetic coil.Therefore, after the transmission is shifted down, the torquedistribution to the rear wheels 21 is increased quickly. During theperiod from the point when the driver has shifted down for accelerationto the point when the driver depresses the accelerator pedal, changingthe frictional engaging force of the coupling 17 is completed. Thus, thedriver depresses the accelerator pedal with the torque distribution tothe rear wheels 21 increased. The driver can therefore acquire thestrong torque feeling soon upon depression of the accelerator pedal.

[0044] The embodiment has the following advantages.

[0045] Prior to determination as to whether the transmission has beenshifted down or not, the ECU 42 determines whether or not the throttleopening Od is greater than the opening threshold value Os. When it isdetermined that the throttle opening Od is greater than the openingthreshold value Os and the transmission has been shifted down, the ECU42 enters the acceleration mode and controls the coupling 17 in such away that the torque distribution to the front wheels 16 and rear wheels21 approaches a uniform state. Therefore, the ECU 42 is prevented fromerroneously determining that a shift-down which was carried out todecelerate the vehicle 11, was a shift-down carried out to acceleratethe vehicle 11. This improves the determination precision.

[0046] Prior to determination as to whether the transmission has beenshifted down or not, the ECU 42 determines whether or not the amount ofchange in the rotational speed of each of the wheels 16, 21 is greaterthan a predetermined change threshold value. When it is determined thatthe amount of change in the rotational speed of at least one wheel 16,21 is greater than the change threshold value, the ECU 42 enters theacceleration mode and controls the coupling 17 in such a way that thetorque distribution to the front wheels 16 and rear wheels 21 approachesa uniform state. Even if it is determined that the amount of change inthe rotational speed of every wheel 16, 21 is equal to or smaller thanthe change threshold value, the ECU 42 enters the acceleration mode whenit is determined that the transmission has been shifted down.

[0047] The embodiment may be modified as follow.

[0048] The electromagnetic clutch mechanism 18 may be replaced with aclutch mechanism, such as a hydraulic clutch mechanism, other than theelectronically controllable electromagnetic clutch mechanism 18.

[0049] The vehicle 11 may have a center differential (differentialcontrol unit). In other words, the present invention may be adapted to afull-time driving system, which always drives four wheels.

[0050] The vehicle 11 may be adapted to a four-wheel drive vehicle whoserear wheels 21 function as main drive wheels and whose front wheels 16function as sub-drive wheels. In this case, the front wheels 16 arecoupled to the engine 12 via the coupling 17 and the rear wheels 21 arecoupled to the engine 12 without the intervention of the coupling 17.

[0051] In the embodiment, the vehicle speed V is calculated based on theaverage of the rotational speeds of the left and right rear wheels 21.However, a vehicle speed sensor (vehicle speed detection device) may beprovided instead to directly detect the vehicle speed V.

[0052] The manual transmission may be replaced with an automatictransmission.

[0053] The torque distribution ratio D_(SUB/MAIN) is not limited to0/100 to 50/50 but may be change between 20/80 to 50/50.

1. A driving force distribution control apparatus for a four-wheel drivevehicle which has an engine, front wheels and rear wheels to be drivenby torque generated by said engine, a coupling capable of changingtorque distribution to the front wheels and rear wheels, and atransmission, the driving force distribution control apparatuscomprising: a controller which controls said coupling in such a way thatthe torque distribution to said front wheels and rear wheels approachesa uniform state when having determined that said transmission has beenshifted down.
 2. The driving force distribution control apparatusaccording to claim 1, further comprising a throttle opening detectiondevice that detects a throttle opening which is a degree of opening of athrottle valve provided in said engine, wherein: said controllercontrols said coupling in such a way that the torque distribution tosaid front wheels and rear wheels approaches a uniform state, whenhaving determined that the detected throttle opening is greater than apredetermined opening threshold value and said transmission has beenshifted down.
 3. The driving force distribution control apparatusaccording to claim 2, wherein said opening threshold value is equivalentto a value of the throttle opening at the time of engine braking.
 4. Thedriving force distribution control apparatus according to claim 1,wherein said controller determines whether or not an amount of change inrotational speed of each of the front wheels and the rear wheels isgreater than a predetermined change threshold value and controls saidcoupling in such a way that the torque distribution to said front wheelsand rear wheels approaches a uniform state when having determined thatthe amount of change in rotational speed of at least one of the frontwheels and the rear wheels is greater than said predetermined changethreshold value.
 5. The driving force distribution control apparatusaccording to claim 1, wherein either said front wheels or said rearwheels are main drive wheels to be coupled to said engine without theintervention of said coupling and the others are sub-drive wheels to becoupled to said engine via said coupling.
 6. The driving forcedistribution control apparatus according to claim 5, wherein saidcontroller controls said coupling in such a way as to increase thetorque distribution to said sub-drive wheels, thereby controlling saidcoupling in such a way that the torque distribution to said front wheelsand rear wheels approaches a uniform state.
 7. The driving forcedistribution control apparatus according to claim 1, further comprisinga gear position detection device that detects a gear position of saidtransmission, wherein: said controller determines whether or not saidtransmission is shifted down based on a change in the detected gearposition.
 8. A driving force distribution control method for afour-wheel drive vehicle which has an engine, front wheels and rearwheels to be driven by torque generated by said engine, a couplingcapable of changing torque distribution to the front wheels and rearwheels, and a transmission, the driving force distribution controlmethod comprising: determining whether or not said transmission has beenshifted down; and controlling said coupling in such a way that thetorque distribution to said front wheels and rear wheels approaches auniform state when it is determined that said transmission has beenshifted down.
 9. The driving force distribution control method accordingto claim 8, further comprising: detecting a throttle opening which is adegree of opening of a throttle valve provided in said engine; andcontrolling said coupling in such a way that the torque distribution tosaid front wheels and rear wheels approaches a uniform state when it isdetermined that the detected throttle opening is greater than apredetermined opening threshold value and said transmission has beenshifted down.
 10. The driving force distribution control methodaccording to claim 8, further comprising: detecting an amount of changein rotational speed of each of the front wheels and the rear wheels; andcontrolling said coupling in such a way that the torque distribution tosaid front wheels and rear wheels approaches a uniform state when it isdetermined that the detected amount of change in rotational speed isgreater than a predetermined change threshold value.